(1) Field of the Invention
This invention relates to a transimpedance amplifier circuit for use as a front end amplifier of a burst-signal adaptable optical receiver, which requires a wide dynamic range, of an optical transmission system in a subscriber's Passive Optical Network (PON) system and so on.
(2) Description of the Related Art
In order to increase a dynamic range in an optical receiver, it has been known to vary a feedback resistance value of a transimpedance amplifier circuit, used as a front end amplifier of a receiving circuit, in accordance with the magnitude of an input signal level. Such technique is disclosed in, for example, Japanese Patent Application Kokai Publication No. Sho-59(1984)-50632.
One exemplary configuration of a conventional transimpedance amplifier circuit of a variable feedback resistance type will now be described. FIG. 1 shows a variable feedback resistance type transimpedance amplifier circuit as disclosed on page 177 of the Fourth Separated Volume of Collected Lecture Theses from the Spring National Convention by the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan in 1992. In this variable feedback resistance type transimpedance amplifier circuit, a light receiving element such as a photo-diode PD is connected between a base of a first transistor Q10 and the ground. A collector of the first transistor Q10 is connected to a base of a second transistor Q20. A diode D9, a resistor R9 and a constant-current source I.sub.ee are connected in series between an emitter of the transistor Q20 and the power source V.sub.ee. Also, a feedback resistor Rf is coupled between the base of the transistor Q10 and the connection node between the resistor R9 and the constant-current source I.sub.ee. A diode D1 for varying a feedback resistance value is connected in parallel to the feedback resistor Rf and the resistor R9, thereby constituting a variable feedback resistor circuit. One input terminal of an output buffer 10 is connected to the emitter of the transistor Q20, whilst the other input terminal thereof is connected to a reference voltage generating circuit 20. An output terminal of this output buffer 10 is connected to an output terminal OUT of the transimpedance amplifier circuit. A current flowing through the light receiving diode PD increases in proportion to an increase in the intensity or level of an optical input signal, and hence a current to be inputted to the transimpedance amplifier circuit is increased.
When the diode D1 turns on according to the increase of the input current to the transimpedance amplifier circuit, the transimpedance is decreased. As a result, the deterioration of an output waveform normally caused by a large input level is prevented. According to this method, when the diode D1 is turned on, a phase margin of the feedback circuit is decreased. Therefore, it is necessary to increase a collector current of the amplification stage transistor (Q10) in order to ensure a sufficient phase margin. This results in an increase of shot noise due to a base current, which leads to the deterioration of the receiving sensitivity.
In order to solve this problem, it is necessary to add a function wherein a gain of the amplification stage transistor Q20 is reduced while the diode D1 is being turned on, so that a sufficient phase margin can be ensured even when the diode D1 is turned on. Another transimpedance amplifier, which is provided with such function and fabricated with a GaAs-FET process, has already been realized. Such amplifier is disclosed in Technical Study Report CS92-10, pp. 61-68 published by the Institute of Electronics, Information and Communication Engineers of Japan. FIG. 2 shows a circuit configuration of this transimpedance amplifier. This transimpedance amplifier circuit comprises: an input signal amplifying section 6; a feedback resistor variable circuit 3 wherein an FET is connected in parallel to a feedback resistor Rf and a gate voltage of this FET is controlled by an output voltage of the amplifier circuit; and a load resistor variable circuit 4 which reduces a gain of the amplifying section 6 in accordance with the increase in the output voltage. In this circuit configuration, it is possible to reduce both the transimpedance and the gain of the amplification stage in response to an increase of the output voltage. Hence, the deterioration of an output waveform caused by a large input level can be prevented and, at the same time, a sufficient phase margin can be ensured.
However, in order to realize such a circuit in the form of an integrated circuit, it is necessary to use a GaAs-FET which has a low mass-producibility or a Si-MOSFET which suffers from significantly large variations in threshold voltages. Therefore, it is difficult to achieve good yield, which greatly increases costs.